A conventional antitheft system for a vehicle employs a technique to disable an engine from being started by an invalid engine start key, in which only when an ID code recorded in an engine start key coincides with an ID code recorded on a vehicle side (immobilizer control unit), the starting of the engine is allowed (see, for example, JP-A-2001-146148, JP-A-10-82223, JP-A-10-129421, JP-A-2000-335370). In this system, even if a duplicate key is formed, since the ID codes do not coincide with each other, it becomes impossible to start the engine of the vehicle.
Incidentally, some work machines (vehicles) such as backhoes exemplified as rotary work machines are provided with engines as power sources to cause the work machines run. Also in this type of work machine, it is conceivable that an antitheft device similar to the case of an automobile is incorporated, and also in this case, when the engine is started by a valid engine start key, the starting of the engine is allowed, and when the engine is started by an invalid start key, the starting of the engine can be prevented. Further, different from the automobile, in the case of the work machine such as the backhoe, a common engine key is generally used as an engine key to start engines of plural work machines, so that one engine key can start the engines of plural work machines. Accordingly, it is important to protect the work machines from theft by ID codes.
However, in the conventional system, in the case where an engine start key registered in an immobilizer control unit is lost, since it is impossible to newly form an engine start key for enabling the starting of the engine of the vehicle with using a commercially available engine key, the whole antitheft device must be replaced, and this is not economical and inconvenient. On the other hand, if an engine start key for enabling the starting of the engine of the vehicle can be newly easily formed with using a commercially available engine key, there arises a problem that the antitheft function is impaired.
In view of the above problems, the present invention has an object to provide an antitheft system for a vehicle in which even in the case where an engine start key is lost, the engine can be started economically and conveniently without ruining the antitheft function.